For radiographic tests, a method of obtaining two-dimensional images called an “X-ray planar detector” whereby electrons generated by a two-dimensional array of X-ray sensors are electronically swept and measured using switching elements is conceived, instead of a conventional radiographic method using X-ray films. In this apparatus, storing electrons and switching elements make it possible to manufacture capacitors all together using thin-film transistor (TFT) manufacturing process used to manufacture liquid crystal displays, etc.
With respect to the method of obtaining an amount of charge corresponding to an amount of X-ray, the following two methods are conceived for the X-ray planar detector.
A first method is called an “indirect method”, whereby X-rays are converted to light by a fluorescent body (scintillator) which converts X-rays to light and then this light is converted to photoelectrons by a photodiode to be stored.
A second method is called a “direct method” whereby a semiconductor for directly converting X-rays to light is used and electric charge output from the semiconductor is stored in a capacitor.
Of these methods, the indirect method can manufacture a fluorescent body which converts X-rays to light and a TFT panel made up of a photodiode, capacitor and read gate separately because there is no electric connection between the fluorescent body and TFT panel, therefore, the indirect method has an advantage of providing an easy way of manufacturing. On the other hand, the indirect method converts X-rays to light once and therefore has a disadvantage of causing deterioration of conversion efficiency and deterioration of resolution caused by scattering of light.
In contrast, the direct method needs to insert a high purity semiconductor conversion material with little leakage current between a voltage application electrode (normally, the surface side of a detector) and a charge collecting electrode (may also serve as one side of a capacitor), and therefore the direct method is said to require a high-level technology from both aspects of creation of materials and manufacturing. However, since the direct method intrinsically has a high efficiency in conversion from X-ray to charge and allows the charge generated by direct conversion from X-rays to be collected by the charge collecting electrode located right below without being scattered by an electric field applied between the voltage application electrode and charge collecting electrode, the direct method has a great advantage of obtaining high resolution, clear image.
With an X-ray planar detector using the direct method, an amount of charge proportional to the amount of incident X-rays is generated inside a semiconductor conversion layer, and it is necessary to apply a relatively large voltage to the voltage application electrode to collect this charge. For this reason, a large amount of charge is stored in a cell which has a large amount of incident X-rays, and as a result, there is a possibility that an extremely high voltage exceeding this withstand voltage may be applied to the capacitor and read gate.
As a technology to protect the capacitor and read gate from such a danger, a method of inserting a Zener diode between the charge collecting electrode and grounding potential (ground), which turns to be conducted at a predetermined voltage not greater than a breakdown voltage of these elements, is known (see Japanese Patent Laid-Open No. 10-10237).
An outline of an X-ray sensor array using this conventional technology will be explained using FIG. 10 and FIG. 11. FIG. 10 is a block diagram of an X-ray sensor array made up of 4 pixels×4 pixels. In the figure, a pixel cell 103 corresponds to a 1-pixel X-ray sensor. Each pixel cell 103 is constructed of a read gate 100, a Zener diode 101 and a charge collecting electrode 102 which also servers as a charge storage capacitor. Though not shown in the figure, an X-ray semiconductor conversion material is laid on the surface of the charge collecting electrode, where X-rays are converted to charge and stored in the charge collecting electrode 102. The charge stored in the charge collecting electrode is read under the control of a gate driver 104, sent to a read amplifier 105 and converted to an electric signal. This signal is scanned by a multiplexer 106 and output to the outside successively.
FIG. 11 shows an equivalent circuit diagram of one pixel cell and the read amplifier 105 connected to this pixel cell. In the same figure, reference numeral 110 denotes a charge collecting power supply to collect charge generated in the X-ray conversion semiconductor layer into the charge collecting electrode 102, 111 denotes an equivalent capacitance (sensor section capacitance) of the X-ray conversion section, 112 denotes a storage electrode capacitance corresponding to the charge collecting electrode 102, 101 denotes a Zener diode and 100 denotes a read gate. Furthermore, reference numeral 114 denotes an OP amplifier and 115 denotes a feedback capacitance and the charge collected through the read gate 100 by these components into the storage electrode capacitance 112 is converted to a voltage signal, which becomes an output signal.
One end of the Zener diode 101 of each cell is grounded and the breakdown voltage of this Zener diode is set to a value lower than the breakdown voltage of the read gate 100.
On the other hand, the potential on the read amplifier side of the read gate 100 is almost fixed to approximately the grounding potential and a voltage approximately equivalent to the voltage applied to the storage charge capacitance 112 is applied between the source and drain of the read gate 100.
Irradiation of X-rays increases charge stored in the storage electrode capacitance 112 and when the breakdown voltage of the Zener diode 101 is exceeded, the charge collected from the X-ray sensor section thereafter is passed to the ground as a breakdown current of the Zener diode. For this reason, the voltage applied between the source and drain of the read gate 100 is always suppressed to the breakdown voltage of the Zener diode 101 or below preventing the read gate from being broken by excessive irradiation of X-rays.
The X-ray sensor array according to the conventional technology is as described above, but using the Zener diode to clamp the voltage applied to the read gate requires an extra process of forming a PN junction to be added to a normal TFT manufacturing process. Moreover, the breakdown voltage of the Zener diode is determined by the concentration of impurities of the semiconductors forming the junction (to be exact, higher concentration of the two), and therefore controlling the breakdown voltage to the source-drain withstand voltage of the read gate or below requires the concentration of impurities to be controlled, which further requires an extra process.